To maximize performance and communication capacity of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention in recent years. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. In this case, the weight can be expressed by a weight vector or a weight matrix, which is respectively referred to as a precoding vector or a precoding matrix.
The spatial multiplexing is classified into single-user spatial multiplexing and multi-user spatial multiplexing. The single-user spatial multiplexing is also referred to as a single user MIMO (SU-MIMO). The multi-user spatial multiplexing is also referred to as a spatial division multiple access (SDMA) or a multi user MIMO (MU-MIMO). A capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel can be decomposed into independent channels. If the number of Tx antennas is Nt, and the number of Rx antennas is Nr, then the number of independent channels is Ni where Ni≦min{Nt, Mr}. Each independent channel can be referred to as a spatial layer. A rank represents the number of non-zero eigen-values of the MIMO channel and can be defined as the number of spatial streams that can be multiplexed.
The MIMO technique includes a codebook-based precoding scheme. The codebook-based precoding scheme performs data precoding by using a precoding matrix which is the most similar to a MIMO channel among predetermined precoding matrices. The use of the codebook-based precoding scheme can result in decrease of overhead since a precoding matrix indicator (PMI) can be transmitted as feedback data. The codebook consists of a codebook set capable of representing a spatial channel. The number of antennas has to be increased to obtain a higher data transfer rate. The greater the number of antennas, the greater the number of codebook sets used to configure the codebook.
Recently, a user equipment having four antennas is taken into account. Therefore, there is a need for a codebook applicable to the increased number of antennas of the user equipment. A new codebook is designed by considering the following aspects. (1) A signal having a low peak-to-average power ratio (PAPR) in an uplink shall be able to be transmitted, and power shall be effectively used when transmitting the low-PAPR signal. (2) A signal of some antennas may be transmitted with power lower than actual power due to an obstacle located right before such as a hand gripping situation, and a preferable antenna for this case shall be able to be used selectively. (3) When applying a pre-defined downlink codebook to an uplink scenario, a problem caused by a limited maximum output of the user equipment shall be taken input account. In a low geometry, a signal is transmitted by increasing output power, and due to a limited output of a power amplifier of the user equipment, a signal having a lower PAPR can be transmitted with effective power by using a defined downlink codebook. However, Tx symbols are additionally used due to elements of a row of a conventional codebook, and thus the PAPR may increase. As such, the codebook of which the PAPR is increasing is not suitable in uplink transmission in which power is limited.
Accordingly, there is a need to design a codebook suitable for uplink transmission depending on the number of antennas of a user equipment in a multiple antenna system.